This invention relates to a synchronous detector circuit and, more particularly, to a synchronous detector circuit which is capable of detecting the video information signal which is provided at the output of the video IF stage in a television receiver.
In a television receiver, the received composite television signal is converted into an intermediate frequency (IF) signal. Since the television signal is transmitted as a vestigial sideband signal, the IF signal includes a video signal component modulated onto a video IF carrier, a chrominance component modulated onto an IF chrominance subcarrier and a sound component modulated onto an IF sound carrier. The video signal, which includes the luminance component and the horizontal synchronizing signal, is amplitude-modulated onto the IF video carrier; and the sound signal is frequency-modulated onto the IF sound carrier. Typically, in the IF stage of a television receiver, the IF amplifier is adjusted to limit application of the modulated sound frequencies, but to provide a higher gain for the video IF carrier and, thus, a higher gain for some of the video IF sidebands. This has the effect of substantially attenuating the sound component, whereby a relatively simple detector circuit, such as a diode detector, can be used to demodulate the IF video signal.
However, because of the low efficiency of a diode detector, and because of unwanted intermodulation products in the IF frequency band, alternative video detector circuits have been proposed. One type of detector which has been considered for use in a television receiver is the synchronous detector. A synchronous detector generally is a multiplier circuit which multiplies an amplitude-modulated signal with the carrier of the amplitude-modulated signal. The product of this multiplication is the information signal which had been used to amplitude-modulate the carrier. If the carrier which is supplied to the synchronous detector is in exact phase with the amplitude-modulated signal, a highly accurate demodulation operation is achieved.
Synchronous detectors have been used satisfactorily at relatively low frequencies. For example, the relatively low IF frequencies in radio reception lend themselves to accurate detection of the modulating signal. However, synchronous detectors heretofore have offered less than satisfactory results when used to detect a video IF signal.
In one type of synchronous video detector, the carrier, also known as the switching carrier, which is multiplied with the IF signal is derived from the IF signal by passing the output of, for example, the video IF amplifier through a limiter and a bandpass filter so as to recover essentially only the IF carrier. This recovered IF carrier then is multiplied with the video IF signal produced by the video IF amplifier. Since the video IF signal is a vestigial sideband signal, various sideband components, including the IF chrominance subcarrier and the IF sound carrier combine with the IF video carrier to vary the phase thereof. The limiter, which is connected ahead, or upstream of the band-pass filter, generally cannot remove these interfering components. Consequently, the phase of the derived switching carrier is not stable. Furthermore, unpredictable phase modulations in this switching carrier are a function of the amplitude modulations on the IF video carrier. Thus, there is an unstable, changing phase relation between the IF video signal and the switching carrier which is derived therefrom. Because of this changing phase relation, synchronous detection of the video signal is difficult, and the original video signal often cannot be recovered with high fidelity.
In one type of synchronous video detector, such as described in U.S. Pat. No. 3,697,685, the limiter and bandpass filter are combined into a single circuit. The limiter is formed as a differential amplifier having a pair of output terminals. A tuned LC circuit is connected across the differential amplifier output terminals, and a pair of parallel-connected, oppositely-poled diodes also are connected across these output terminals. Although this circuit is of a relatively simple configuration and can be fabricated as an integrated circuit, it suffers from the same disadvantage as described above. That is, the phase of the derived switching carrier varies with amplitude modulations in the IF video signal primarily because of the inability of the limiter to suppress various IF sideband components sufficiently. Furthermore, in this type of limiter-filter combination, the Q of the band-pass filter is relatively low. This reduces the efficiency of the limiter-filter combination in deriving the switching carrier. Amplitude detection of the video signal is not achieved with high fidelity, resulting in some degradation of the ultimately reproduced video picture.